WASHINGTON — Just days after President Trump spoke of a “running war’’ with the media, his chief White House strategist, Stephen K. Bannon, ratcheted up the attacks, arguing that news organizations had been “humiliated” by the election outcome and repeatedly describing the media as “the opposition party” of the current administration.

“The media should be embarrassed and humiliated and keep its mouth shut and just listen for a while,” Mr. Bannon said in an interview on Wednesday.

“I want you to quote this,” Mr. Bannon added. “The media here is the opposition party. They don’t understand this country. They still do not understand why Donald Trump is the president of the United States.”

The scathing assessment — delivered by one of Mr. Trump’s most trusted and influential advisers, in the first days of his presidency — comes at a moment of high tension between the news media and the administration, with skirmishes over the size of Mr. Trump’s inaugural crowd and the president’s false claims that millions of illegal votes by undocumented immigrants swayed the popular vote against him.

Mr. Bannon, who rarely grants interviews to journalists outside of Breitbart News, the provocative right-wing website he ran until August, was echoing comments by Mr. Trump last weekend, when the president said he was in “a running war” with the media and called journalists “among the most dishonest people on earth.” Mr. Bannon’s remarks added to the growing acrimony between the press and a president who made attacks on the media a rallying point of his election campaign.

Among Mr. Trump’s advisers in the White House, Mr. Bannon is responsible for putting into action the nationalist vision that Mr. Trump channeled during the later months of the campaign, one that stemmed from Mr. Bannon himself. And in many ways Mr. Trump has acted on that vision during his first week in office — from the description of “American carnage” he laid out in his inauguration speech to a series of executive actions outlining policies on trade agreements, immigration and the building of a border wall.

Mr. Bannon is one of the strongest forces in an administration with competing power centers. A savvy manipulator of the press, and a proud provocateur, he was among the few advisers in Mr. Trump’s circle who were said to have urged Sean Spicer, the new press secretary, to give a confrontational, emotional statement to a shocked West Wing briefing room on Saturday, when the White House disputed news reports about the size of the inauguration crowd. He shares Mr. Trump’s view that the news media misunderstood the movement that the president rode into office.

Speaking by telephone on Wednesday, Mr. Bannon delivered a broad indictment of the news media as being biased against Mr. Trump and out of touch with the American public. That is an argument familiar to readers of Breitbart and followers of personalities friendly to Mr. Trump, like Sean Hannity of Fox News.

“The elite media got it dead wrong, 100 percent dead wrong,” Mr. Bannon said of the election, calling it “a humiliating defeat that they will never wash away, that will always be there.”

“The mainstream media has not fired or terminated anyone associated with following our campaign,” Mr. Bannon said. “Look at the Twitter feeds of those people: They were outright activists of the Clinton campaign.” (He did not name specific reporters or editors.)

“That’s why you have no power,” he added. “You were humiliated.”

Mr. Bannon spoke in blunt but calm tones, peppered with profanity, and humorously referred to himself as “Darth Vader.” He said, with ironic relish, that Mr. Trump was elected by a surge of support from “the working-class hobbits and deplorables.”

The conversation was initiated by Mr. Bannon to offer praise for Mr. Spicer, who has been criticized this week for making false claims at the White House podium about attendance at Mr. Trump’s inaugural, for calling reporters dishonest and lecturing them about what stories to write, and for failing to disavow Mr. Trump’s lie about widespread voter fraud in the election.

Asked if he was concerned that Mr. Spicer had lost credibility with the news media, Mr. Bannon chortled. “Are you kidding me?” he said. “We think that’s a badge of honor. ‘Questioning his integrity’ — are you kidding me? The media has zero integrity, zero intelligence, and no hard work.”

“You’re the opposition party,” he said. “Not the Democratic Party. You’re the opposition party. The media’s the opposition party.”

Journalists reacted with alarm and defiance to Mr. Bannon’s comments. “What country are we living in?” Christiane Amanpour, the CNN correspondent, wrote on Twitter.

“We are not the opposition,’’ Stephen Engelberg, editor in chief of the nonprofit news organization ProPublica, wrote in an email. “We are part of an essential function in any democracy.” He added that ProPublica had no intention of “shutting up in response to this or any other president’s demand.”

“We are here to tell the truth and we intend to continue doing so, regardless of how badly some might want us to parrot ‘alternative facts,’” Mr. Engelberg said.

Mr. Bannon mostly referred to the “elite” or “mainstream” media, but he cited The New York Times and The Washington Post by name.

“The paper of record for our beloved republic, The New York Times, should be absolutely ashamed and humiliated,” Mr. Bannon said. “They got it 100 percent wrong.”

He added that he has been a reader of The Times for most of his adult life.

Maggie Haberman and Sydney Ember contributed reporting from New York.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Top Strategist Casts Media as ‘Opposition Party’. Order Reprints | Today’s Paper | Subscribe